1. Technical Field
The present invention relates generally to a visible light communication apparatus and dimming control technology and, more particularly, to a visible light communication apparatus and a visible light communication method using the same, which are applied to dimming control in Variable Pulse Position Modulation (VPPM) mode and can perform minute dimming control.
2. Description of the Related Art
Visible light is light having a wavelength in the range visible to the human eye among electromagnetic waves, and the wavelength ranges from 380 nm to 780 nm.
In visible light, variations in characteristics resulting from differences in wavelength are exhibited in the form of colors, and the wavelength decreases across the spectrum from red to violet. Light having a wavelength longer than that of red color is referred to as infrared light, and light having a wavelength shorter than that of violet is referred to as ultraviolet light. In the case of monochromatic light, 700˜610 nm corresponds to red, 610˜590 nm corresponds to orange, 590˜570 nm corresponds to yellow, 570˜500 nm corresponds to green, 500˜450 nm corresponds to blue, and 450˜400 nm corresponds to violet. A variety of colors can be represented by mixing colors having the wavelengths.
Visible light is visible to humans, unlike infrared light or ultraviolet light. Illumination which emits visible light should satisfy a variety of requirements such as accurate color representation. One of these requirements is little flickering.
Since humans cannot perceive 200 or more flickers per second, illumination apparatuses using Light Emitting Diodes (LEDs) having fast flicker performance control flickering using Pulse Width Modulation (PWM) in order to increase the lifespan thereof and save energy.
Visible Light Communication (VLC) is a wireless communication technology using a wavelength in the range of 380 nm to 780 nm. The visible light communication standardization process is being conducted within the IEEE 802.15 Wireless Personal Area Network (WPAN) Working Group. In Korea, the Telecommunications Technology Association (TTA) is operating a visible light communication working group.
For visible light communication, several modulation methods are being discussed. One of these methods is On-Off Keying (00K). On-off keying is a modulation method in which the case where a signal level is high represents ‘1’ and the case where a signal level is low represents ‘0’, or the case where a signal level is high represents ‘0’ and the case where a signal level is low represents ‘1.’ This on-off keying is chiefly used in optical communication because modulation signals can be directly converted into the flickers of light.
In addition to on-off keying, Pulse Position Modulation (PPM), Pulse Amplitude Modulation (PAM), and sub-carrier 4PPM (SC-4PPM) in which a carrier is combined with a PPM signal are being considered as modulation methods for visible light communication.
Methods of controlling the dimming of LED illumination apparatuses include a variety of methods, such as a method of varying the amount of current flowing through LEDs by varying a voltage applied to a simple variable resistor, and a method of controlling periods in which LEDs remain turned on by varying the pulse width of a voltage applied to the LEDs.
In illumination apparatuses using LEDs, the dimming of illumination is controlled using Pulse Width Modulation (PWM) which achieves the highest efficiency and minute current control. PWM controls the luminance of light by controlling periods in which LEDs remain turned on using a method of controlling the ratio of signal-on to signal-off. That is, when the period in which LEDs remain turned on is long within a unit time, LEDs emit bright light. In contrast, when the period in which LEDs remain turned on is short within a unit time, the illumination of the LEDs becomes dark.
The visible light communication technology includes a Variable PPM (VPPM) technology that enables both communication and dimming at the same time. This technology enables both communication and dimming at the same time by varying the length of an interval in which a light source remains turned on within a single symbol of PPM.
However, minute dimming using only a pulse width is problematic in that it is difficult to implement it and in that a lot of cost and time are required even though it can be implemented. Furthermore, when a signal is transmitted with its pulse width adjusted minutely, it is difficult to decode the signal at a receiver.
Accordingly, there is an urgent need for a new dimming control technology which is capable of more simply and effectively controlling brightness in visible light communication.